Despite the growing availability of alternative energy sources, the energy needs of most of the world are primarily met by the combustion of fossil fuels such as coal, oil, and natural gas. Among other byproducts, such combustion produces carbon dioxide (CO2), the release of which into the environment is becoming increasingly regulated. CO2 emissions may be mitigated, at least in part, by capturing CO2 directly from large sources of emissions, such as generating plants that combust fossil fuels as a power source. Rather than simply venting CO2 into the atmosphere, CO2 can be removed from flue gases using a CO2 absorber. The captured CO2 may then be utilized in other processes or sequestered. Similar capture methods can also be applied to other industrial processes that generate significant amounts of CO2 including, for example, ammonia production, fermentation processes, removal of CO2 from natural gas or biofuel products, and so forth.
CO2 is generally separated from a gas mixture by absorption. The flue gas, for example, may be a flue gas generated by a boiler that produces steam for a power plant. In a typical process the gas mixture is passed through an absorption column where the gas is exposed to a capture medium that absorbs some or all of the CO2 component of the mixture. Typically, this absorbent is in a liquid phase and is often an aqueous solvent that contains one or more amine compounds. Such a process is sometimes referred to as wet scrubbing. The gas mixture is passed through the absorption solution at pressure and temperature conditions that permit absorption of substantially all the CO2 into the absorption solvent. The CO2-lean gas mixture emerges at the top of the absorption column and may be directed for further processing as necessary. The absorption solvent, which is now rich in CO2 (i.e. a rich solvent), exits from the bottom of the absorption column, and is then subjected to a stripping process to remove the CO2 and regenerate an absorption solvent that is lean in CO2 (i.e. a lean solvent).
Regeneration of the lean solvent typically involves heating the rich solvent to reduce the solubility of CO2. To ensure complete or near complete removal of CO2, the rich solvent may undergo successive cycles of reheating. In a typical solvent regeneration process, rich solvent is introduced into a regeneration column at a high temperature. This elevated temperature is maintained by a reboiler. At these elevated temperatures, the rich solvent releases absorbed CO2. The regenerated lean solvent may be collected from the bottom of the regeneration column for reuse in the absorption column, while a gas phase containing the stripped CO2 (along with water) is collected from the top of the regeneration column. This gas phase may be passed through a condenser system that condenses water vapor and returns the liquid to the regeneration column. The released CO2 may then be collected for reutilization or sequestration.
As noted above, the energy requirements for existing absorption and recovery processes can be significant, in large part due to the heat required for stripping CO2 from the rich solvent. Because this heat is typically derived from steam that would otherwise be used for power production, the heat requirements of the reboiler can reduce net power production. Attempts have been made to reduce this burden by redirecting heat from other plant processes. For example, U.S. Pat. No. 5,344,627 (to Fujii et al) discusses discharging steam from a high pressure turbine and directing it to an auxiliary turbine that powers a compressor used to liquefy recovered CO2. The steam discharge of this auxiliary turbine provides heat for a reboiler. In WIPO publ. no. WO2011/073671, Hume and Kuczynska describe a similar approach in which steam discharged from a high pressure turbine is utilized to drive a back pressure turbine, which in turn drives a compressor used to compress CO2 recovered from the rich solvent. This compression produces heat, which along with heat from the exhaust of the back pressure turbine, is supplied to the reboiler. However, since the discharge from high pressure turbines is typically utilized in lower pressure turbines to generate additional power, these approaches still directly impact power generation.
A similar approach is described in U.S. pat. publ. no. 2010/0050637 (to Yamashita et al), which discloses diverting a portion of the steam input to a low pressure turbine to drive an auxiliary turbine, and utilizing the steam exhaust from this auxiliary turbine as a heat source for a reboiler. Since this directly impacts the power output of the low pressure turbine, however, power generation is still impacted. An alternative approach is disclosed in U.S. pat. publ. no. 2010/0050637 (to Yamashita et al), where a reboiler is supplied with heat via a heating medium, which is in turn receives heat from the flue that transports the initial waste gases via a heat exchanger. It is, however, unclear if such a source can provide sufficient heat at the proper temperature for the needs of the reboiler.
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Thus, there is still a need for systems and methods that reduce the energy requirements for regeneration of solvents used in CO2 capture from flue gases, particularly in power generation plants that rely on fossil fuel combustion.